[TowerTalk] Capacitors To Tune 160 Vertical

[David Gilbert]

I don't understand that comment.  Murphy's Law has nothing to do with it 
... Ohms Law does.  As N6RK said, where high voltage DC is present, 
capacitors in series are typically bridged by high value parallel 
resistors to make the voltage divider dependent upon the resistors 
instead of the much higher (and indeterminate) DC resistance of the 
dielectric.  For RF, the impedance of the capacitors themselves provides 
that same shunting action as far as the bulk resistance of the 
dielectric is concerned.  Look at any model of a capacitor and you'll 
see an ideal capacitor shunted by a high resistance, with bits of series 
resistance and inductance that affect Q.  Jim and Rick are right ... the 
RF voltage will divide among series capacitors almost strictly as a 
function of their capacitance.

Besides, you have a similar situation to worry about with parallel 
capacitors except the culprit is current instead of voltage.  At higher 
frequencies where layout (inductance) is an issue, I'd bet unequal 
current sharing among paralleled capacitors blows more of them than does 
unequal voltage dividing among series capacitors.

All that being said, not many inexpensive RF capacitors handle several 
amps very well, so for high power applications we probably end up with 
some configuration that includes both series and parallel devices ... 
and I'd probably shoot for more margin on the parallel portion than on 
the series portion.

73,
Dave   AB7E


Bill Turner wrote:
>
>
> Murphy is waiting. :-)
>
> 73, Bill W6WRT
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>